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A molecular force probe


Force probes1 allow reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed. Unlike strain energy2, approaches based on restoring force allow quantitative molecular understanding3 of phenomena as diverse as translation of microscopic objects by reacting molecules4,5,6, crack propagation7,8 and mechanosensing9. Conceptually, localized reactions offer the best opportunity to gain fundamental insights into how rates vary with restoring forces, but such reactions are particularly difficult to study systematically using microscopic force probes10,11,12,13,14. Here, we show how a molecular force probe, stiff stilbene, simplifies force spectroscopy of localized reactions. We illustrate the capabilities of our approach by validating the central postulate of chemomechanical kinetics15—force lowers the activation barrier proportionally to the difference in a single internuclear distance between the ground and transition states projected on the force vector—on a paradigmatic unimolecular reaction: concerted dissociation of the C–C bond.

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Figure 1: Microscopic versus molecular force probes.
Figure 2: The experimental system used to illustrate the use of a molecular force probe, stiff stilbene (red), for studies of the chemomechanical kinetics of localized reactions.
Figure 3: Kinetic and force data for macrocycles 1–9.
Figure 4: Validation of the single-coordinate model of chemomechanical kinetics.

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The authors would like to thank J. I. Brauman, M. Gruebele, J. F. Hartwig, J. S. Moore, T. J. Martinez, K. S. Suslick and G. M. Whitesides for helpful discussions. The work was supported by the National Science Foundation (NSF) CAREER Award (CHE-0748281), the US Air Force Office of the Scientific Research Young Investigator Award (FA9550-08-1-0072), the American Chemical Society Petroleum Research Fund (48454-AC3 and 43354-G3) and the University of Illinois. T.K. thanks the Office of the Naval Research and the NSF for pre-doctoral fellowships. We gratefully acknowledge grants of computational time by the Air Force Research Lab Major Shared Resource Center and the National Center for Supercomputing Applications.

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Q.Y. designed and synthesized the macrocycles, Z.H. measured the kinetics, T.K. synthesized the macrocycles, D.K. carried out DFT computations of thermodynamic parameters and reaction coordinates, J.C. carried out preliminary MM and DFT calculations, R.B. designed the experiments, developed methods for calculating the restoring forces, analysed the results and wrote the paper.

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Correspondence to Roman Boulatov.

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Yang, QZ., Huang, Z., Kucharski, T. et al. A molecular force probe. Nature Nanotech 4, 302–306 (2009).

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